REDUCTION OF EDDY CURRENTS DURING FLOW ENCODED MAGNETIC RESONANCE IMAGING

Abstract

In a method and magnetic resonance (MR) apparatus for establishing imaging sequence parameter values with a reduced eddy current formation for flow-encoded magnetic resonance imaging, a number of different flow-encoded candidate raw datasets are acquired by executing a flow-encoded gradient measurement sequence with different imaging sequence parameter values from a test or calibration region of an examination object. Flow-encoded candidate image datasets are reconstructed from the different flow-encoded candidate raw datasets. A flow-encoded candidate image dataset is selected as a function of a background phase contrast established in a phase-contrast image assigned to the respective flow-encoded candidate image dataset. The imaging sequence parameter values assigned to the flow-encoded candidate image dataset are selected as parameter values for an imaging sequence for subsequent diagnostic flow-encoded magnetic resonance imaging.

Claims

1. A method for establishing imaging sequence parameter values that operate a magnetic resonance (MR) data acquisition scanner with a reduced eddy current formation for flow-encoded MR imaging, said method comprising: operating said MR data acquisition scanner to execute a plurality of flow-encoded gradient measurement sequences, respectively with different imaging sequence parameter values, in order to acquire a plurality of different flow-encoded candidate raw datasets from a test or calibration region of an examination object in the MR data acquisition scanner; providing said different flow-encoded candidate raw datasets to a reconstruction processor and, in said reconstruction processor, reconstructing flow-encoded image datasets respectively from said different flow-encoded candidate raw datasets; in a selection processor, selecting a flow-encoded candidate image dataset, from among said flow-encoded candidate image datasets, dependent on a background phase contrast established in a phase-contrast image assigned to one of said flow-encoded candidate image datasets; and in a defining processor, defining imaging sequence parameter values assigned to the selected flow-encoded candidate image dataset, as parameter values for subsequent execution of a diagnostic flow-encoded gradient MR sequence, and providing said parameter values for said diagnostic flow-encoded gradient MR sequence in an electronic form configured to operate said MR data acquisition scanner.

2. A method as claimed in claim 1 comprising, in said selection processor, selecting the flow-encoded candidate image dataset for which the assigned phase-contrast image exhibits a weakest background phase contrast in said test or calibration region.

3. A method as claimed in claim 1 comprising using a region of the examination object having a stationary phase signal as said test or calibration region.

4. A method as claimed in claim 3 comprising, in said selection processor, identifying said test or candidate region having a stationary phase signal automatically.

5. A method as claimed in claim 1 comprising receiving a manual entry into said selection processor that establishes said test or candidate region.

6. A method as claimed in claim 1 comprising generating said image sequence parameter values from the group consisting of a first gradient moment of flow-encoded gradients, gradient moments higher than the first gradient moment of the flow-encoded gradients, a course of flow-encoded gradients, a slice orientation from which flow-encoded MR data are to be acquired, an echo time at which MR signals are read out in said flow-encoded gradient MR sequence, and a repetition time of said flow-encoded gradient MR sequence.

7. A method as claimed in claim 1 comprising operating said MR data acquisition scanner to acquire said flow-encoded candidate raw datasets with at least one of a spatial resolution or a temporal resolution that is lower than a spatial resolution or a temporal resolution in said diagnostic flow-encoded gradient MR sequence.

8. A method as claimed in claim 1 comprising displaying the phase-contrast images assigned to the respective flow-encoded candidate image datasets at a display in communication with said selection processor, and selecting the selected flow-encoded candidate image dataset dependent on a manual entry made in selection processor after a visual evaluation of the displayed phase-contrast images.

9. A method as claimed in claim 1 comprising selecting the selected flow-encoded image dataset by automatically establishing an average value of the background phase contrast in said test or calibration region.

10. A method for acquiring flow-encoded magnetic resonance image (MR) data comprising: operating an MR data acquisition scanner to execute a plurality of flow-encoded gradient measurement sequences, respectively with different imaging sequence parameter values, in order to acquire a plurality of different flow-encoded candidate raw datasets from a test or calibration region of an examination object in the MR data acquisition scanner; providing said different flow-encoded candidate raw datasets to a reconstruction processor and, in said reconstruction processor, reconstructing flow-encoded image datasets respectively from said different flow-encoded candidate raw datasets; in a selection processor, selecting a flow-encoded candidate image dataset, from among said flow-encoded candidate image datasets, dependent on a background phase contrast established in a phase-contrast image assigned to one of said flow-encoded candidate image datasets; in a defining processor, defining imaging sequence parameter values assigned to the selected flow-encoded candidate image dataset, as parameter values for subsequent execution of a diagnostic flow-encoded gradient MR sequence, and providing said parameter values for said diagnostic flow-encoded gradient MR sequence in an electronic form configured to operate said MR data acquisition scanner; operating the MR data acquisition scanner to execute said diagnostic flow-encoded gradient MR sequence to acquire diagnostic flow-encoded raw data from a diagnostic region of the examination object; and providing the diagnostic flow-encoded raw data to said reconstruction processor and, in said reconstruction processor, reconstructing diagnostic flow-encoded image data from the acquired diagnostic flow-encoded raw data, and making the reconstructed diagnostic flow-encoded image data available from the reconstruction computer in electronic form as a data file.

11. A control computer for a magnetic resonance (MR) apparatus comprising: an MR data acquisition scanner with a radio frequency (RF) transmitter, a gradient coil system, and an RF receiver; an RF transmit controller that operates said RF transmitter to radiate RF pulses; a gradient system interface configured to activate said gradient coil arrangement; an RF reception controller that receives raw data from said RF detector; a control computer configured to operate said RF transmit controller, said gradient system interface, and said RF reception controller to execute a plurality of flow-encoded gradient measurement sequences, respectively with different imaging sequence parameter values, in order to acquire a plurality of different flow-encoded candidate raw datasets from a test or calibration region of an examination object in the MR data acquisition scanner; a reconstruction processor provided with said different flow-encoded candidate raw datasets, said processor being configured to reconstruct flow-encoded image datasets respectively from said different flow-encoded candidate raw datasets; a selection processor configured to select a flow-encoded candidate image dataset, from among said flow-encoded candidate image datasets, dependent on a background phase contrast established in a phase-contrast image assigned to one of said flow-encoded candidate image datasets; and a defining processor configured to define imaging sequence parameter values assigned to the selected flow-encoded candidate image dataset, as parameter values for subsequent execution of a diagnostic flow-encoded gradient MR sequence, and to provide said parameter values for said diagnostic flow-encoded gradient MR sequence in an electronic form configured to operate said MR data acquisition scanner.

12. A magnetic resonance (MR) apparatus comprising: an MR data acquisition scanner; a control computer configured to operate said MR data acquisition scanner to execute a plurality of flow-encoded gradient measurement sequences, respectively with different imaging sequence parameter values, in order to acquire a plurality of different flow-encoded candidate raw datasets from a test or calibration region of an examination object in the MR data acquisition scanner; a reconstruction processor provided with said different flow-encoded candidate raw datasets, said processor being configured to reconstruct flow-encoded image datasets respectively from said different flow-encoded candidate raw datasets; a selection processor configured to select a flow-encoded candidate image dataset, from among said flow-encoded candidate image datasets, dependent on a background phase contrast established in a phase-contrast image assigned to one of said flow-encoded candidate image datasets; and a defining processor configured to define imaging sequence parameter values assigned to the selected flow-encoded candidate image dataset, as parameter values for subsequent execution of a diagnostic flow-encoded magnetic resonance imaging, and to provide said parameter values for said diagnostic flow-encoded magnetic resonance imaging in an electronic form configured to operate said MR data acquisition scanner.

13. A non-transitory, computer-readable data storage medium encoded with programming instructions, said storage medium being loaded into a computer system of a magnetic resonance (MR) apparatus that comprises an MR data acquisition scanner, said programming instructions causing said computer system to: operate said MR data acquisition scanner to execute a plurality of flow-encoded gradient measurement sequences, respectively with different imaging sequence parameter values, in order to acquire a plurality of different flow-encoded candidate raw datasets from a test or calibration region of an examination object in the MR data acquisition scanner; reconstruct flow-encoded image datasets respectively from said different flow-encoded candidate raw datasets; select a flow-encoded candidate image dataset, from among said flow-encoded candidate image datasets, dependent on a background phase contrast established in a phase-contrast image assigned to one of said flow-encoded candidate image datasets; and define imaging sequence parameter values assigned to the selected flow-encoded candidate image dataset, as parameter values for subsequent execution of a diagnostic flow-encoded magnetic resonance imaging, and provide said parameter values for said diagnostic flow-encoded magnetic resonance imaging in an electronic form configured to operate said MR data acquisition scanner.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0041] FIG. 1 shows a pulse diagram, which illustrates a test or calibration sequence as a reference measurement for flow-encoded magnetic resonance imaging.

[0042] FIG. 2 shows a pulse diagram, which illustrates a sequence for measuring a velocity in the z-direction within the framework of diagnostic flow-encoded magnetic resonance imaging.

[0043] FIG. 3 shows a flowchart of the basic steps of a method for acquisition of flow-encoded magnetic resonance image data in accordance with an exemplary embodiment of the invention.

[0044] FIG. 4 is a schematic illustration of a magnetic resonance apparatus in accordance with an exemplary embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0045] FIG. 1 shows a pulse diagram, which illustrates a sequence for a reference measurement for flow-encoded magnetic resonance imaging. In a first line a radio-frequency excitation pulse RF is shown, with which spins are excited in a region to be examined. Simultaneously with the radio-frequency excitation pulse RF, a slice-selection gradient G.sub.z shown in the second line is also applied in the z-direction. The slice-selection gradient G.sub.z is designed so that its zero gradient moment M.sub.0 and its first gradient moment M.sub.1 each have the value 0. The zero gradient moment M.sub.0 is produced as the integral of the slice-selection gradient G.sub.z over time:

[00001]$\begin{array}{cc}{M}_0={\int }_0^t.Math.G_z\left(t^{\prime }\right).Math..Math.t^{\prime }.& \left(1\right)\end{array}$

[0046] The first gradient moment M.sub.1 is produced as the integral of the product of the slice-selection gradient G.sub.z and the location z(t) over time:

[00002]$\begin{array}{cc}{M}_0={\int }_0^t.Math.z\left(t^{\prime }\right).Math.G_z\left(t^{\prime }\right).Math..Math.t^{\prime }.& \left(2\right)\end{array}$

[0047] The sequence shown in FIG. 1 can be used for a measurement of a reference phase, from which velocity-related phase values can be subtracted. A read-out gradient G.sub.x is shown in a third line in FIG. 1, with which a first magnetic resonance signal or read-out signal S.sub.1 (see bottom line in FIG. 1) is read out. Also shown in FIG. 1 is a gradient G.sub.y in the phase-encoding direction, with which a non-velocity-dependent phase of a read-out signal is influenced.

[0048] FIG. 2 shows a pulse diagram, which illustrates a sequence for a measurement for diagnostic flow-encoded magnetic resonance imaging. In a first line a radio-frequency excitation pulse RF is shown, with which spins are excited in a region to be examined. Simultaneously with the radio-frequency excitation pulse RF, a slice-selection gradient G.sub.z shown in the second line will also be applied in the z-direction. Unlike in the part sequence in FIG. 1, in FIG. 2 the slice-selection gradient G.sub.z is supplemented by a proportion, so that although the magnetic moment zero M.sub.0 of the gradient G.sub.z has the value 0, as it does in the part sequence in FIG. 1, the first magnetic moment M.sub.1 is not equal to 0 however.

[0049] For an excitation with this part sequence a phase of a second read-out signal S.sub.2 with a velocity-dependent proportion is created, which can be measured as the phase of the second read-out signal S.sub.2. By subtracting the phase of the first read-out signal S.sub.1 established with the aid of the first part sequence from the phase of the second read-out signal S.sub.2 established with the second part sequence, the velocity-dependent proportion of the signal phase in the z-direction, also referred to as the phase-contrast or flow contrast in the z-direction, is established. In a similar way the velocity-dependent phase signals in the x- and y-direction can also be measured, from which once again the stationary phase established with the first read-out signal S.sub.1 must be subtracted. Basically velocity-dependent phase signals only occur in part regions of an examination region in which moving spins, i.e. moving material, is present, however as a result of the eddy current fields or eddy current described at the outset, phase-contrasts are also measured in regions without moving spins. In order to minimize this effect, a method for acquisition of flow-encoded magnetic resonance image data in accordance with an exemplary embodiment of the invention is shown in FIG. 3.

[0050] FIG. 3 shows a flowchart 300 with the basic steps of such a method for acquisition of flow-encoded magnetic resonance image data. In a step 3.1, in a test-mode recording, also known as a preliminary recording, first of all a number of different flow-encoded candidate raw datasets K-FRDS.sub.i are acquired with the aid of a phase-contrast measurement sequence PMS with different imaging sequence parameter values BS-PW.sub.i from a part region TB of an examination object O. In this case the part region TB is preferably selected so that above all stationary tissue, in which no moving spins are present, is present in the part region.

[0051] To this end individual parameters, such as the echo time, the repetition time of the sequence PMS or the gradient form of the respective flow-encoding gradients, are varied between the recording of the individual candidate raw datasets K-FRDS.sub.i. Depending on the variation of the parameters, the effects created as a result of the eddy current fields on the phase signals are characterized differently. In order to establish these effects, in a step 3.11, flow-encoded candidate image datasets K-FBDS.sub.i are reconstructed on the basis of the different flow-encoded candidate raw datasets K-FRDS.sub.i. The reconstruction is done with a usual reconstruction method for reconstruction of image data, which is based on a Fourier transformation for example. Subsequently, in a step 3.111, the phase-contrast images PK-B.sub.i assigned to the reconstructed flow-encoded candidate image datasets K-FBDS.sub.i are examined and compared in respect of a characterization of the eddy-current-related phase contrast. Preferably the phase-contrast image PK-B.sub.k or the associated phase-contrast image dataset K-FBDS.sub.k is now established, for which the lowest eddy-current-related phase contrast occurs. Subsequently, in a step 3.IV, the imaging sequence parameter values BS-PW.sub.k assigned to the phase-contrast image PK-B.sub.k established is defined as parameter values BS-PW for subsequent flow-encoded magnetic resonance imaging.

[0052] In a step IV the actual flow-encoded magnetic resonance imaging, for example of a heart in motion, is started. Here the flow-encoded raw data FRD is acquired with the aid of a phase-contrast measurement sequence BS with the defined imaging sequence parameter values BS-PW from a region of an examination object O to be examined, in this case, the heart of a patient. Finally, in a step 3.VI, flow-encoded image data FBD are reconstructed on the basis of the acquired flow-encoded raw data FRD. Artifacts resulting from eddy current fields are minimized in the images created on the basis of the flow-encoded image data FBD.

[0053] FIG. 4 shows a schematic illustration of an inventive magnetic resonance system 1 (abbreviated below to “MR system”), which has a control computer 13 in accordance with an exemplary embodiment of the invention. The magnetic resonance system includes the actual magnetic resonance scanner 2 with an examination volume 3 or patient tunnel, into which an examination object O (a patient or test subject) in which the examination object, for example a specific organ, is located, can be moved on a bed 8.

[0054] The magnetic resonance scanner 2 is equipped in the usual way with a basic field magnet 4, a gradient system 6 and an RF transmit antenna system 5 and an RF receive antenna system 7. In the exemplary embodiment, the RF transmit antenna system 5 involves a whole-body coil permanently installed in the magnetic resonance scanner 2, while the RF receive antenna system 7 includes local coils to be arranged on the patient or test subject (symbolized in FIG. 4 by just one single local coil). Basically, the whole-body coil can also be used as the RF receive antenna system and the local coils can be used as the RF transmit antenna system, provided these coils are able to be switched in different operating modes in each case.

[0055] The MR system 1 also has the central control computer 13 already mentioned in accordance with an exemplary embodiment of the invention, which is used for controlling the MR system 1. This central control computer 13 has a sequence controller 14 for pulse sequence control. The sequence of radio-frequency pulses (RF pulses) and of gradient pulses is controlled by this unit as a function of a selected imaging sequence. Such an imaging sequence can be predetermined, for example, within a measurement or control protocol. Usually different control protocols are stored in a memory 19 for different measurements and can be selected by a user (and where necessary changed if required) and can then be used for carrying out the measurement.

[0056] For output of the individual RF pulses, the central control computer 13 has a radio-frequency transmit system 15, which creates the RF pulses, amplifies them and injects them via a suitable interface (not shown in detail) into the RF transmit antenna system 5. For control of the gradient coils of the gradient system 6, the control computer 13 has a gradient system interface 16. The sequence controller 14 communicates with the radio-frequency transmit system 15 in a suitable way, e.g. by sending out sequence control data SD, and with the gradient system interface 16 for sending out the pulse sequences. The control computer 13 also has a radio-frequency receive system 17 (likewise communicating in a suitable way with the sequence controller 14), in order to acquire magnetic resonance signals, i.e. raw data, received in a coordinated manner from the RF transmit antenna system 7.

[0057] A reconstruction processor 18 accepts the acquired raw data and reconstructs the MR image data therefrom. This image data can then be stored in a memory 19 for example and/or can be further processed, in order to establish optimum imaging parameters BD-PW for phase-contrast imaging in accordance with the method described in conjunction with FIG. 3. To this end reconstructed, flow-encoded candidate image datasets K-FBDS.sub.i are transferred to a selection unit 11, which selects from the flow-encoded candidate image datasets K-FBDS.sub.i that have been reconstructed by the reconstruction processor 18 on the basis of acquired flow-encoded candidate-raw data, a suitable or an optimal flow-encoded candidate image dataset K-FBDS.sub.k. Then, in a definition processor 12—as has already been explained—the imaging sequence parameter values BS-PW assigned to the selected flow-encoded candidate image dataset K-FBDS.sub.k are defined as imaging sequence parameter values BS-PW for a subsequent magnetic resonance image recording of the patient O and transferred to the sequence controller 14 or transferred for storage to the memory 19.

[0058] As an alternative, the selection processor 11 can be integrated into the reconstruction processor 18 here or can be linked externally via a network or the like to the central control computer 13.

[0059] The central controller 13 can be operated via a terminal with an input unit 10 and a display unit 9, with which the entire MR system 1 can also be operated by an operator. MR images can also be displayed on the display unit 9 and during the selection step can be appraised by a user, and by means of the input unit 10, if necessary in combination with the display unit 9, measurements can be planned and started and in particular suitable control protocols with suitable measurement sequences, as explained above, can be selected and modified if necessary.

[0060] Moreover, the inventive MR system 1 and in particular the control computer 13 can have a number of further components, not shown individually here but usually present in such devices, such as a network interface for example, in order to connect the overall system to a network and to be able to exchange raw data and/or also further data, such as patient-relevant data or control protocols, for example.

[0061] How suitable raw data can be acquired by radiating RF pulses and creating gradient fields and how MR images can be reconstructed from the raw data are basically known to those skilled in the art and need not be explained in greater detail herein. Likewise the principles of a wide range of measurement sequences, such as EPI measurement sequences or measurement sequences for creating flow-encoded images, are known to those skilled in the art.

[0062] Although modifications and changes may be suggested by those skilled in the art, it is the intention of the Applicant to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of the Applicant's contribution to the art.